Vacuum skin packaging (VSP) is a process well known in the art using a thermoplastic packaging material to enclose a food product. The terms “vacuum skin packaging” or “VSP” as used herein indicate that the product is packaged under vacuum and the space containing the product is evacuated from gases at the moment of packaging. The top flexible film is also referred to as “skin-forming”, “skin” or “upper” film.
In the vacuum skin packaging process, an article may be placed on a rigid, semi-rigid or flexible support member, that can be flat or shaped, e.g., tray-shaped, bowl-shaped or cup-shaped (called “bottom” web), and the supported article is then passed to a chamber where a “top” web is first drawn upward against a heated dome and then draped down over the article. The movement of the top web is controlled by vacuum and/or air pressure, and in a vacuum skin packaging arrangement, the interior of the container is vacuumed before final welding of the top web to the bottom web. In the VSP process, the upper heated film forms a tight skin around the product and is tightly adhered to the part of the support not covered by the product. Vacuum skin packaging is described in many references, including FR1258357, FR1286018, AU3491504, U.S. Re 30009, U.S. Pat. Nos. 3,574,642, 3,681,092, 3,713,849, 4,055,672, 5,346,735, WO2009141214, EP2722279, EP2459448.
Vacuum skin packaging is commonly employed for packaging food products such as fresh and frozen meat and fish, cheese, processed meat, ready meals and the like. The final package presents a tight fitting, clear package which protects the food article from the external environment.
The demands imposed nowadays on the packaging films used in such vacuum skin packaging applications are particularly high: the films have to stand the heating and stretching conditions within the vacuum chamber of the packaging machine without undergoing excessive softening and perforations, they must be highly formable and have an excellent implosion resistance (as hereinafter defined) in addition of being ovenable, in case of ready-meals packaging.
Good formability is highly desirable in VSP applications to ensure that the heated film adequately conforms to the shape of the packaged product, without leaving pleats on the package surfaces or without forming protruding areas of self-adhesion of the film, at the package corners or sides. This unwanted phenomenon, known as bridging or webbing, can be so marked to extend to separate forming units in the same packaging operation. Obviously, packages showing these defects in the top skin draping are not acceptable for the consumer and therefore they have to be rejected. Other important features of VSP films include optical properties, such as glossiness and haze, which contribute to an attractive package appearance.
Moreover, VSP films must guarantee, in combination with supports of various shape and composition, an appropriate opening force in order to allow the consumer easily opening the package and taking out the packaged product. During storage, in addition to easy openability, seal and package integrity must be kept, in order to effectively preserve the product.
As well known in the art, such performance in terms of easy opening and integrity is particularly difficult to be achieved in a vacuum skin package. In fact, vacuum skin packaging processes do not apply a standard sealing step through a heated sealing bar, as for example in tray lidding packaging processes, but the adhesion of the top to the bottom is achieved through the draping of the heated top onto the entire surface of the bottom not covered by the product after removal of the vacuum from the dome. The final quality and strength of the adhesion are mainly dependent on the temperature achieved by the top, on the sealing properties of the outer surfaces and on the chemical compatibility between the sealant layers of the top and the bottom.
In case of films for vacuum skin ready meals packages, the above requirements of easy openability and hermeticity are even more difficult to be achieved. The seal strength must be adequate to keep the packs hermetic before the heat/microwave treatment step and to allow easy opening by the consumer after that treatment.
For ready meals applications, which necessitate heating or cooking of the food product in conventional or microwave ovens, supports currently used are polyester-based, (e.g. APET, CPET trays), as polyesters are endowed with the required thermal properties.
However, directly sealing conventional VSP top films to polyester based supports has always been difficult due to the scarce sealing characteristics of polyesters.
For this reason, polyester supports have generally been coated with sealants or sealable layer in order to be properly sealed to a top film.
In the alternative, top films have been subjected to superficial treatments (e.g. corona treatments) in order to improve sealability as described for instance in EP1728731, commented below.
It would be desirable to make available to the market VSP top films having very good sealability on polyester supports, but not only, and possibly being ovenable, particularly microwaveable.
Another important feature of VSP packages, particularly for fresh meat (including poultry) or fish, is the capacity of controlling the drip loss, i.e. the liquid purge or juices, which exude from the packaged food over time. Such purge not only make the package quite unpleasant at first sight for the final consumer, but also represents a net weight loss of the product to be consumed.
EP1728731, in the name of Curwood, relates to vacuum skin packages formed from a thermoplastic film and a polyester substrate. In order to get a peelable seal between the film and the substrate in a vacuum skin package, EP '731 suggests to perform a surface-treatment to the exterior surface of the film structure thus reaching a surface tension of between 36-60 dynes/cm in order to obtain seal force values in the seal between the top film and the substrate from 0.5 to 6 lb./in. (0.09-1.08 kg/cm).
The top web layers construction exemplified in EP '731 is ethylene vinyl acetate/ionomer/modified polyethylene/ethylene vinyl alcohol/modified polyethylene/ionomer/low density polyethylene. This document is not concerned with the problem of product drip loss in the final package.
A top web having this composition is available on the market and has been tested by the Applicant as hereinafter described (see example C8 comparative film in the experimental part).
The document WO2009032627 in the name of DuPont describes packaging films comprising a polyester substrate and a (co)polyester based sealant layer. These films are not cross-linked. This document is not concerned with the problem of drip loss in VSP packages.
The document WO2015107127A1 in the name of Cryovac, relates to heat-shrinkable films for manufacturing flexible containers (bags) and not to thermoformable films suitable for VSP applications. Accordingly, this document does not provide any teaching relating to drip retention capacity or seal-peel performance in VSP packages. The VSP films presently on the market, such as those described in EP1728731, still need to be improved in term of drip loss retention.
In conclusion, there is still the need to provide VSP films showing an optimized balance of properties, especially the best mix of mechanics, optics and formability, being sealable on supports of various chemical nature, especially on polyesters, and possibly being ovenable, particularly microwaveable. It is desirable that these films can provide VSP packages which are self-venting, easy openable even after heat treatments, and, especially, endowed with an improved drip retention capacity.